The present invention relates to a magnetoresistive element exhibiting magnetoresistance effect and to devices utilizing the magnetoresistive element such as a magnetic thin film memory and a magnetoresistance sensor. More particularly, it relates to a magnetoresistive element of a basically three-layered or five-layered structure which magnetic layers sandwich a nonmagnetic layer.
A phenomenon that a substance varies the electrical resistivity under the application of an external magnetic field is called "magnetoresistance effect" (hereinafter referred to as "MR effect"), and semiconductors and magnetic materials have been known to exhibit this effect.
Among these materials, common ferromagnetic materials exhibit the so-called "anisotropic magnetoresistance effect" (hereinafter referred to as "AMR effect") wherein when .theta. is the angle formed between a magnetization direction and a current direction, the electrical resistivity changes according to a factor of cos.sup.2 .theta.. Ni--Fe alloys are known as materials exhibiting a large AMR effect, which offer the "megnetoresistance ratio" (hereinafter referred to as "MR ratio") of about 3%. Conventional magnetoresistive elements (MR elements) utilize the AMR effect and offer a small MR ratio though they are operative in response to a relatively low magnetic field. Accordingly, there has been a great desire to develop a MR element offering a large MR ratio.
Recently a multilayered film in which magnetic layers 71 's and nonmagnetic layers 72 's are alternately stacked (refer to FIG. 20) has been found to exhibit a MR effect larger than the AMR effect and has been attracting considerable attention. Such a large MR effect is termed "giant magnetoresistance effect" (hereinafter referred to as "GMR effect"). Unlike the AMR effect, this GMR effect is independent of a current direction and is developed by the relative angle formed between the respective magnetizations of adjacent magnetic layers. The resistance assumes a maximum when the two magnetizations are oriented antiparallel to each other and a minimum when they are oriented parallel to each other. Herein, the term "antiparallel" means to indicate an alignment where the two magnetizations are oriented in opposite directions, while the term "parallel" means to indicate an alignment where they are oriented in the same direction.
A Fe/Cr multilayered film is known as an MR element utilizing the GMR effect (refer to, for example, Phys.Rev.Lett., Vol. 61, No. 21, 1988, pp. 2472-2475). In this Fe/Cr multilayered film the antiferromagnetic interaction between magnetic layers (Fe layers) 71's sandwiching a nonmagnetic layer (Cr layer) 72 develops the GMR effect by offering two alignments of antiparallel under a magnetic field being applied set at 0 and parallel under a magnetic field being applied set at a magnitude where magnetization is saturated. However, a problem exists in this multilayered film that the application of a magnetic field (saturation magnetic field) of 10 kOe or more (at room temperature) is required because of the interaction between the magnetic layers and this becomes a great hindrance in practical use.
A NiFe/Cu/Co/Cu multilayered film is also known as a MR element utilizing the GMR effect (refer to, for example, Journal of The Physical Society of Japan, Vol. 59, No. 9. September 1990, pp. 3061-3064). This multilayered film utilizes the difference in coercivity between two kinds of magnetic films to develop the GMR effect. Specifically, a nonmagnetic layer (Cu layer) is made sufficiently thick, for example, about 5 nm to substantially eliminate the interaction between magnetic layers, and a soft magnetic layer (NiFe layer) and a hard magnetic layer (Co layer) realize antiparallel alignment of magnetizations on the basis of the magnitude of a magnetic field to be applied. In this multilayered film, however, the nonmagnetic layer cannot be made so thin because the interaction between the magnetic layers needs to be eliminated. This results in a limited MR ratio of this multilayered film. Further, a change in resistance depends on the magnetization process of the soft and hard magnetic layers and, hence, cannot be positively controlled by a magnetic field applied. In addition, it is desired that the magnetoresistance curve (MR curve) of a MR element be changeable in configuration in accordance with applications of the MR element such as in a magnetic thin film memory and a magnetoresistance sensor (hereinafter referred to as "MR sensor").
A MR sensor disclosed in Japanese Unexamined Patent Publication No. 358310/1992 comprises a three-layered MR element comprised of the first and second ferromagnetic materials which are separated with a thin layer of a nonmagnetic metal. Such a MR element is designed so that the respective magnetization directions of the first and second ferromagnetic thin film layers would cross each other at right angles when a magnetic field applied thereto is set at 0. This MR sensor is described to have a certain inclination of a change in resistance due to rotation of the magnetization direction of the ferromagnetic material when a magnetic field applied is in the proximity of 0. Further, this sensor is characterized by obtaining the MR effect as the sum of the AMR effect and the GMR effect. The sensor, however, offers a small MR ratio though it is suitable as a sensor in view of its characteristics.
The aforementioned conventional single-layered MR element utilizing the AMR effect has a problem of a small rate of change in electrical resistance.
The aforementioned multilayered MR element such as made of Fe/Cr which utilizes the GMR effect requires a large magnetic field, as large as several kilo oersteds or more, to be applied and, hence, has a difficulty in practical use. Alternatively, the multilayered MR element such as made of NiFe/Cu/Co/Cu which utilizes the GMR effect based on the difference in coercivity between magnetic layers suffers a problem of a limited MR ratio in terms of absolute value since the nonmagnetic layer cannot be made so thin.
It is, therefore, an object of the present invention to overcome the foregoing problems and to provide a MR element for use in a magnetic thin film memory, which element utilizes a GMR effect offering a large rate of change in electrical resistance with a small change in the magnetic field, and a magnetic thin film memory employing such a MR element.
Another object of the present invention is to provide an MR element which utilizes a GMR effect offering a large rate of change in electrical resistance with a small change in the magnetic field, and a MR sensor employing such a MR element.
These and other objects will become apparent from the description hereinafter.